1. Field of the Invention
This invention relates in general to subsea completions, interventions, and workovers and, in particular, to a system and method for an umbilical-less subsea test tree control system.
2. Brief Description of Related Art
In some interventions, completions, and workovers, a horizontal christmas tree is used as part of the completion. The horizontal christmas tree includes a subsea control module that supplies hydraulic or electric power to operations below the mudline. During completion and workover operations, a riser will extend from the horizontal tree to a rig or ship at the surface. An intervention or workover stack, including a tubing hanger, universal running tool, and subsea test tree, will often be run down the riser to interact with the horizontal tree. Hydraulic and/or electrical umbilicals are run within the riser to provide hydraulic and/or electric power to the intervention or workover stack. A riser control module may be run with the intervention or workover stack to the horizontal tree level to control operation of the intervention or workover stack. The riser control module requires separate system control equipment at the surface that is connected to the riser control module through the hydraulic and electric umbilicals.
As the water depth increases, direct hydraulic control during the running of tubing and well workover is no longer practical due to the unacceptable hydraulic response times. As a result most all deepwater control systems employ electro hydraulic multiplex (EH MUX) equipment where hydraulic energy is stored in accumulators close to the user, usually with the riser control module, and actuated by electrical signal from the surface. As an alternate to a multiplex solution, a direct electric signal can also be sent directly to each function. Such systems have been developed and are currently in use on deepwater wells around the world. Hydraulic umbilicals will still be run within the riser to maintain hydraulic pressure within the accumulators.
Progressively, in deeper water, an umbilical that is strapped to the landing string, feeds the hydraulic accumulators, and transmits control signals and electricity to the control pod is becoming a capital and operating expenditure cost driver. The deeper the water, the longer the umbilical, which not only drives the umbilical cost, but also adds cost to storage reels and rigging equipment needed at the surface to support the umbilical. Furthermore, the increased overall umbilical weight and size will also impact the capacity of the drill rig. This is the case for both open-water and in-riser systems.
Operationally, strapping the umbilical to the landing string during running and retrieval is a time consuming task. From a safety perspective, manually fastening the umbilical to the riser string using riser clamps is also undesirable because it requires increased worker interaction with equipment around the well opening. In addition, increasing water depths increase the risk of snagging and damage to the umbilical during running and retrieval. This also increases the operational risk of losing items downhole.
An emerging option for deepwater well completion is using a surface blow out preventer (SBOP) on a dynamically positioned drill rig. Combined with a reduced diameter marine riser, from 21″ to 16″ or even 14″, as well as the elimination of choke and kill lines, the depth rating of existing rigs can be increased and capital saved. Wells have already been successfully drilled using a SBOP in this way. In such a development, an extreme premium is placed on real estate inside the smaller marine riser. This available space is further reduced by the need for a landing string to support the intervention or workover stack at the horizontal subsea tree. This further reduces the space for umbilicals. In addition, an SHOP will require a slick joint in the landing riser at the rig SBOP location. This means that the umbilical will need to be terminated at the slick joint or somehow sealed within the slick joint in order to ensure a pressure containing seal around the riser. This further complicates the intervention or workover operations, increasing costs and risks to worker safety. Therefore a subsea test tree control system for use in interventions, workovers, and completions that may be used without hydraulic and/or electrical umbilicals in the riser is desirable.